1. Field of the Invention
The present invention relates to an exhaust gas purification system for an engine that injection-supplies a liquid reducing agent into an exhaust gas flowing on an upstream side of a reduction catalyst in the exhaust system of the engine to reduce NOx in the exhaust gas from the engine thereby purifying the exhaust gas before being exhausted into the atmosphere. More particularly, the invention relates to an apparatus for judging a condition of injection of reducing agent (it will be hereinunder referred to as “injection-condition judging apparatus” for the sake of brevity throughout the disclosure.) used in an exhaust gas purification system that monitors whether or not a liquid reducing agent is being normally injection-supplied.
2. Description of the Related Art
As an exhaust gas purification system which removes the NOx contained in the exhaust gas discharged from an engine, such as a diesel engine and a gasoline engine, an exhaust gas purification system has been proposed, which injection supplies a liquid reducing agent in a required amount corresponding to an engine operating state onto the upstream side of a reduction catalyst disposed in an exhaust system of an engine, to thereby create a catalytic reduction reaction between the NOx in the exhaust gas and the liquid reducing agent in order to purify the NOx to harmless elements (for example, refer to Japanese Unexamined Patent Publication No. 2000-27627). Here in the reduction reaction, ammonia, which has good reactivity with NOx, is used as the reducing agent, and for the liquid reducing agent, a urea water solution serving as a generation source of ammonia, is used. As the reducing agent, it can be considered to use not only ammonia, but also an ammonia water, a hydrocarbon such as gas oil, and alcohol.
Incidentally, in this type of exhaust gas purification system, it is necessary to monitor whether or not the liquid reducing agent is being normally injection-supplied in the required amount corresponding to the engine operating state. In a case where the liquid reducing agent is not being normally injection-supplied, for example, taking as one typical example a system that uses a urea water solution as the liquid reducing agent, this gives rise to problems such that if an injection-supply amount of the urea water solution is less than a specified amount, the NOx exhaust amount increases, while on the other hand, if the injection-supply amount of the urea water solution is greater than the specified amount, excessive ammonia, which is not used for the reduction reaction of the NOx, is discharged.